Their server settings (SRV records, redirects etc.) are now totally broken (no .well-known, SRV/TXT records point to broken /begenda/dav/users path which always returns 403). So I don't know whether we should update this in the documentation, because it looks like a (temporary) error on their side.

Unfortunately, we have never received any response from GMX upon our contact requests, so I guess chances are not too high this time.